Power supply

ABSTRACT

A rechargeable power supply ( 10 ) comprises at least one a rechargeable cell ( 12 ) connectable to a power source ( 14 ), a Zener diode ( 20 ) connected to the at least one cell ( 12 ), and a bipolar transistor or FET ( 22 ) for selectively disconnecting the Zener diode ( 20 ) from the cell ( 12 ) thereby to reduce any discharge of the cell ( 12 ) through the voltage protection means ( 20 ). The Zener diode ( 20 ) and the transistor ( 22 ) are connected together in series and jointly connected across the cell in parallel.

[0001] The present invention relates to a power supply and particularly,but not exclusively, to a rechargeable power supply for poweringelectronic equipment.

[0002] Rechargeable power supplies generally consist of a number ofrechargeable batteries or cells arranged in series, in parallel or in aseries/parallel combination. It is common to provide Zener diodesconnected in parallel across each cell to prevent the recharging voltagesupplied to the cell from exceeding a predetermined level, which couldresult in damage to the cell, and to allow the power supply to continueto supply power to any connected equipment in the event of one or moreof the cells failing in an open circuit condition.

[0003] In such an arrangement, the Zener diodes act as voltageregulators whereby if the voltage applied to the cell exceeds a certainlevel, break down of the Zener diode will occur causing current to leakthrough the Zener diode in the reverse direction. As the applied voltageincreases the current leaking through the Zener diode also increases.This has the effect of clamping the voltage over the cell to apredetermined level.

[0004] To ensure that the cell is not subjected to a voltage higher thanits maximum rating, the Zener diode used must be capable of leaking areverse current which is at least equal to the charging current of thepower supply at a voltage which is less than or equal to the cell'smaximum voltage rating. However, Zener diodes generally do not have astepped on-off break down characteristic and thus permit leakage currentthrough the diode, to greater or lesser extent, over a range ofvoltages. Thus, a Zener diode having a 2.5 mA leakage current at abreakover voltage of 3.3 V may still permit 50% of that current to passthrough at an applied reverse-biased voltage of 3.2 volts. Consequently,once the power supply is fully charged and the charging current removed,the leakage current through the Zener diodes will continue until thevoltage over the cell has been reduced to a point where the Zener diodehas zero leakage current. During this time, the leakage current throughthe Zener diode will cause discharging of the cell.

[0005] Since rechargeable cells are effective for only a limited numberof charge/discharge cycles, this number usually being a function of thepercentage depth of each cycle, the discharge effect of the Zener diodeon the cell means that the potential life of the cell, and thereforepower supply as a whole, is significantly reduced, both in terms of eachcharge/discharge cycle and the overall life of the power supply. This isa particular problem in low power applications (for example smokedetectors/alarms), where the leakage current through the Zener diodes isoften a high multiple of the supply current required to power theequipment. This has the effect of reducing the life of the rechargeablepower supply to a fraction of its potential life.

[0006] The present invention seeks to provide an improved power supply.

[0007] Accordingly, the present invention provides a rechargeable powersupply comprising:

[0008] a rechargeable cell connectable to a power source;

[0009] voltage protection means connected to said cell; and

[0010] disconnecting means for selectively disconnecting said voltageprotection means from said cell thereby to reduce any discharge of saidcell through said voltage protection means.

[0011] The present invention will now be described, by way of exampleonly, with reference to the accompanying drawings in which:

[0012]FIG. 1 is an electrical circuit diagram of a first form of powersupply according to the invention;

[0013]FIG. 2 is an electrical circuit diagram of a second form of powersupply according to the invention;

[0014]FIG. 3 is an electrical circuit diagram of a third form of powersupply according to the invention; and

[0015]FIG. 4 is an electrical circuit diagram of a fourth form of powersupply according to the invention

[0016] Referring to FIG. 1, a preferred form of power supply accordingto the invention is shown generally at 10. The power supply 10 comprisesa single battery or cell 12, the positive terminal of the cell 12 beingconnected to a power source or charging current source in the form of apower rail 14 via a diode 15 and a parallel combination of a resistor 16and a Schottky diode 18. The negative terminal of the cell 12 isconnected to the earth or zero volt rail 17 of the charging currentsource.

[0017] The cell 12 has a series combination of voltage protection means,in the form of a Zener diode 20, and disconnecting means, in the form ofa field effect transistor (FET) 22 connected in parallel across the cell12. The Zener diode 20 is connected across the cell 12 in a reversebiased direction, i.e. with its cathode connected to the positiveterminal of the cell 12. The anode of the Zener diode 20 is connected tothe drain electrode of the FET 22 whilst the source electrode of the FET22 is connected to the negative terminal of the cell 12.

[0018] It will be appreciated that any electrically equivalentconnection of the Zener diode and FET, for example with the anode of theZener diode being connected to the negative terminal of the cell 12, thecathode of the Zener diode being connected to the source of the FET andthe drain of the FET being connected to the positive terminal of thecell, will function equally well.

[0019] The gate electrode of the FET 22 is connected directly to thepower rail 14 of the charging current source.

[0020] In operation, when a voltage is applied to the power rail 14 ofthe charging current source, thereby to charge the cell, current flowsthrough the diode 15 and resistor 16 and through the cell 12 to earth.The purpose of the resistor 16 is to limit the current through the cellto a value appropriate for charging the cell. At the same time, thevoltage on the power rail 14 is applied to the gate electrode of the FET22 which is thereby switched on. This connects the Zener diode 20 acrossthe cell 12.

[0021] As the cell 12 is charged, the voltage across the cell rises. Ifthe voltage across the cell 12, and thus across the Zener diode 20,rises above the breakover voltage of the Zener diode 20, the Zener diode20 will break down and begin to conduct leakage current therethrough Theflow of leakage current through the Zener diode 20 prevents the voltageacross the cell 12 from rising further and thus the voltage iseffectively clamped at or around the breakover voltage of the Zenerdiode. It is normal, therefore, for the breakover voltage of the Zenerdiode to be chosen to correspond substantially to the maximum voltagerating of the cell. It will be appreciated, therefore, that the presenceof the Zener diode prevents overcharging of the cell 12.

[0022] When the cell is sufficiently charged and the voltage on thepower rail 14 from the charging current source is switched off, currentflows from the charged cell 12 through the Schottky diode 18 to thepower rail 14 and out to any connected electronic equipment or circuit.Current is prevented from returning to the charging current source bythe diode 15. The Schottky diode 18 is included to provide a lowimpedance path between the cell 12 and the connected equipment orcircuit However, it is not essential to the invention and its inclusionis entirely optional.

[0023] As described above, however, with the Zener diode 20 connectedacross the cell 12, leakage current will continue to flow through theZener diode 20 thus gradually discharging the cell 12, possibly at arate greater than that caused by the electronic equipment itself. Toprevent this, therefore, when the voltage on the power rail 14 from thecharge current source is switched off, the voltage applied to the gateelectrode of the FET 22 is reduced to zero such that the FET 22 isswitched off and presents an open circuit. The Zener diode 20 istherefore effectively disconnected from the cell 12 and thus no leakagecurrent can flow therethrough. All current from the cell 12 is thusapplied to the electronic equipment or circuit and any unwanteddischarging to the Zener diode 20 is effectively eliminated.

[0024]FIG. 2 illustrates a similar arrangement but where the powersupply comprises three, series-connected cells 12, hereafter termedcollectively as the “battery”. In this embodiment, each cell 12 in thebattery has a Zener diode 20 connected in parallel with it and a FET 22connected in series with the Zener diode. It is clear from the drawingsthat the arrangement within the dashed box 10 in FIG. 1 is repeated foreach cell in the battery of FIG. 2.

[0025] In this embodiment, when the voltage across any one of the cells12 in the battery exceeds the breakover voltage of the cell's respectiveZener diode 20, leakage current will be passed through the Zener diodeand prevent the voltage on the cell from rising further.

[0026] In FIG. 3, the power supply is shown having a differentarrangement of cells 12. In this case, the battery is comprised of 4cells 12 in a two by two, series-parallel arrangement. Each pair ofparallel connected cells 12 is provided with a respective Zener diode 20and FET 22 connected in parallel over the cell pair.

[0027] The mode of operation of the embodiment of FIG. 3 is similar tothat of FIGS. 1 and 2 in that if the voltage over any one of the cells12 rises above the breakover voltage of the respective Zener diode 20,then leakage current through the Zener diode will prevent that voltagefrom rising any further.

[0028] In FIG. 4, a stepped arrangement of voltage protection isprovided. In this embodiment, the battery of the power supply comprisesthree cells 12 a, 12 b, 12 c connected in series. The series combinationof cells 12 a and 12 b has a first Zener diode 20(i) and FET 22(i)connected in parallel thereover. The series combination of cells 12 band 12 c has a second Zener diode 20(ii) and FET 22(ii) connected inparallel thereover while the series combination of all three cells 12 a,12 b, 12 c have a third Zener diode 20(iii) and FET 22(iii) connected inparallel thereover.

[0029] As will be clearly understood by those skilled in the art, if thevoltage over the series combination of cells 12 a and 12 b exceeds thebreakover voltage of Zener diode 20(i), then leakage current throughZener diode 20(i) will prevent the voltage over the cells from risingany further. Similarly, if the voltage over cells 12 b and 12 c exceedsthe breakover voltage of Zener diode 20(i) then leakage current throughZener diode 20(ii) will prevent the voltage over cells 12 b and 12 cfrom increasing any further. Finally, if the combined voltage over allthree cells exceeds the breakover voltage of Zener diode 20(ii) thenthis diode will break down and leakage current through Zener diode20(iii) will prevent the voltage over cells 12 a, 12 b and 12 c fromincreasing any further.

[0030] In all illustrated embodiments, the voltage protection providedby the Zener diodes 20 is only effective when the respective FET 22connected to the Zener diode is switched on thereby to connect the Zenerdiode across the respective cell or cells. Each FET is switched on onlywhen the power rail 14 of the charge current source is energised, forexample when the cells 12 are being charged. When the power rail 14 isde-energised, each FET 22 will switch off thus disconnecting therespective Zener diode from the respective cell or cells 12, thuspreventing any leakage current from passing through the Zener diode andthus avoiding any unwanted discharging of the cell or cells.

[0031] It will be appreciated that the present invention allows the safecharging of one or more rechargeable cells and avoids thedisadvantageous effects of both overcharging of one or more of the cellsand excessive discharging of the cells during normal operation.

[0032] It will also be appreciated, however, that a number ofmodifications may be made to the invention as desired. For example, theZener diode 20 may be replaced by resistive devices or any other deviceswhich permit a selected level of leakage current to pass therethrough.In addition, the FET's 22 could be replaced by any other form ofelectronic switches or by any other device which upon energising of thepower rail 14, causes the voltage protection means to be connectedacross one or more of the cells. It is even envisaged that a mechanicalarrangement, such as a manually operated switch, may be employed toselectively disconnect the Zener diodes or other voltage protectionmeans from the cell or cells.

[0033] It will be clear that the invention is not limited to anyparticular kind of rechargeable cell which may be, for example,nickel-cadmium (Ni—Cd) “NICAD” cells, nickel metal hydride (NiMH) cellsor any other chemical rechargeable cells or even solid cells (e.g.lithium polymer cells), capacitors or any other type of rechargeabledevice capable of storing and delivering electrical energy.

[0034] It is also possible for the invention easily to be configured formechanical applications such as fuel cells for gas, liquid or other fuelor pressure containers. In this case the voltage protection means may bereplaced by pressure release valves or other such devices.

[0035] In order to allow the power supply to operate in the event of anopen circuit failure of one or more of the cells, conventional diodesmay be permanently connected in parallel across each cell or cell pairto ensure a continuous path around the failed cell or cells. Havingnegligible reverse leakage current, these diodes will not discharge thepower supply in any way. A possible arrangement for such diodes 23 isshown in FIG. 3 whilst in FIG. 4, each FET 22(i, ii, ii) is providedwith an integral diode 24.

1. A rechargeable power supply (10) comprising; at least one arechargeable cell (12) connectable to a power source (14); voltageprotection means (20) connected to said at least one cell; anddisconnecting means (22) for selectively disconnecting said voltageprotection means (20) from said cell (12) thereby to reduce anydischarge of said cell through said voltage protection means (20).
 2. Apower supply as claimed in claim 1 wherein said voltage protection means(20) comprises a Zener diode.
 3. A power supply as claimed in claim 1 orclaim 2 wherein said disconnecting means (22) is a bipolar transistor orFET.
 4. A power supply as claimed in any preceding claim wherein saidvoltage protection means (20) and said disconnecting means (22) areconnected together in series and jointly connected across said cell inparallel.
 5. A power supply as claimed in claim 3 or claim 4 whenappendant to claim 3 wherein the base electrode of the bipolartransistor or the gate electrode of the FET is connectable to said powersource (14) thereby to actuate said disconnecting means.
 6. A powersupply as claimed in any preceding claim further comprising at least onediode (23, 24) connected in parallel with said cell (12) or with saiddisconnecting means (20).
 7. A power supply as claimed in any precedingclaim comprising a battery consisting of a plurality of series, parallelor series-parallel connected cells.
 8. A method of protecting arechargeable cell (12) in a power supply (10), the method comprising:during recharging of said cell, selectively connecting voltageprotection means (20) to said cell thereby to reduce or eliminateovercharging of said cell; and after recharging of said cell,selectively disconnecting said voltage protection means from said cellthereby to reduce or eliminate any discharging of said cell through saidvoltage protection means.